Foil-air bearings are advantageous as they operate efficiently over a large temperature range, have increasing load capacity as rotational speeds increase, can handle severe environmental conditions, and are more reliable than rolling element bearings because they require fewer parts to support the rotating assemblies and do not need lubrication. However, they also require inherently tight tolerance controls on the bearings themselves and their housings in order to maintain a very close alignment between each radial bearing in a rotor system in order to operate effectively. Bearing misalignment can be caused by excessive manufacturing tolerances, operational loads or thermal distortion of the bearing housing, and can result in shaft angular misalignment causing unacceptable whirl instability. Existing foil-air bearings also require a good control of both radial stiffness and damping characteristics of the inner foil support structure.
The use of hydrodynamic fluid journal bearings having compliant foils is known. The inner foils of the bearing are arranged such that the end sections of the foils themselves are more compliant than the central section of the foils, and can deflect independently in order to provide improved conformity to a misaligned rotor supported by the bearing. Strong misalignment forces can thereby be tolerated. U.S. Pat. No. 4,274,683 teaches such a compliant foil bearing. However, these hydrodynamic journal bearings require metallic corrugated foil assemblies that are comprised of several separate sections of foils, which must each be independently flexible and able to deflect different controlled amounts. Introducing independent flexibility to the metallic foils themselves considerably adds to the complexity, and consequently cost, of the bearing. Additionally, as each bearing requires several discrete metallic foil sections having different flexibility, the complexity and manufacturing costs are further increased.
There remains a need to provide a foil-fluid bearing capable of tolerating angular misalignment, without significantly adding complexity to the internal construction of the bearing, and consequently without significantly adding to material and manufacturing costs.